Transmission bandwidth demands in telecommunication networks (e.g., the Internet) appear to be ever increasing and solutions are being sought to support this bandwidth demand. One solution to problem is to use optical networks, where dense wavelength-division-multiplexing (DWDM) technology is used to support the ever-growing demand for higher data rates. Commonly used optical components include optical filters.
An optical filter can be implemented in an optical fiber or in a planar waveguide circuit (PWC). PWC-based optical filters are likely to be significant in future WDM systems and networks. However, typical conventional PWC-based optical filters use special materials such as III-V compound semiconductors (GaAs, InP, AlGaAs, and so on) and LiNiO3 or are mechanical such as micro-electro-mechanical (MEM) structures. These approaches tend to be complex and expensive compared to silicon-based approaches.
On conventional optical filter uses a Fabry-Perot (FP) filter. As is well known, FP filters have two reflective surfaces and a cavity between. A FP filter allows optical signals of the resonant wavelengths to pass through, reflecting signals that are not of the resonant wavelengths. However, a conventional FP filter has multiple transmission peaks with the distance between peaks referred to as the free spectral range (FSR). FP filters achieve relatively narrow pass bands, which are desirable in many optical filter applications, but the multiple transmission peaks may be unsuitable for DWDM applications. The FSR may be decreased by lengthening the distance between the reflective surfaces, but this increases the width of the pass bands. Further, conventional PWC-based FP filters are typically implemented using MEM technology or other relatively complex technology. Thus, a conventional FP filter may not be practical for use in DWDM applications.